Disk-based storage devices such as hard disk drives (HDDs) are used to provide non-volatile data storage in a wide variety of different types of data processing systems. A typical HDD comprises a spindle which holds one or more flat circular storage disks, also referred to as platters. Each storage disk comprises a substrate made from a non-magnetic material, such as aluminum or glass, which is coated with one or more thin layers of magnetic material. In operation, data is read from and written to tracks of the storage disk via a read/write head that is moved precisely across the disk surface by a positioning arm as the disk spins at high speed.
Conventional HDDs typically employ a read/write head position control system in which position error detection fields, generally referred to as “servo” marks, are written at fixed intervals on the storage disk. These servo marks are written only once at drive manufacture, utilizing a servo writer. By way of example, in a wedge servo arrangement, the servo marks are formed in designated radial wedges distributed around the disk. Data wedges between the servo wedges contain multiple data sectors and consume most of the track capacity. A servo algorithm uses the servo marks from the servo wedges to detect head position. No position feedback is available during the data wedge, so the servo algorithm typically must interpolate head position between the detected servo marks. Other arrangements of servo marks are also possible. For example, servo marks may be distributed evenly throughout the disk, rather than organized into servo wedges.
During HDD operation, drive hardware reads the servo marks in order to calculate an estimate of read/write head position error, which is then used in a firmware control loop to maintain the radial position of the read/write head. Because disk space dedicated for servo marks cannot be utilized to store user data, the number of servo marks written on the disk defines a tradeoff between the bandwidth of the read/write head position control loop and the capacity of the drive.
The storage capacity of HDDs continues to increase, and HDDs that can store multiple terabytes (TB) of data are currently available. However, increasing the storage capacity often involves shrinking track dimensions in order to fit more tracks onto each storage disk, such that inter-track interference (ITI) and read/write head position become important performance-limiting issues. Also, read/write head scaling is limited, so eventually the magnetic field used to write one track will impact adjacent tracks and thereby limit track density.
A number of recording techniques have been developed in an attempt to further increase HDD storage capacity. For example, a recording technique known as shingled magnetic recording (SMR) attempts to increase storage capacity of an HDD by “shingling” a given track over a previously written adjacent track on a storage disk. In another recording technique, referred to as bit-patterned media (BPM), high density tracks of magnetic islands are preformed on the surface of the storage disk, and bits of data are written to respective ones of these islands. Nonetheless, ITI and read/write head position remain important performance-limiting issues with these and other HDD recording techniques.
The adverse impact of ITI on HDD performance may be addressed in some cases through application of ITI cancellation techniques upon readout. Such cancellation techniques may involve, for example, performing ITI reduction post-processing on data read from the storage disk. In a technique of this type, information about an interfering data pattern stored on an adjacent track is used to detect ITI-induced read signal noise, and to cancel that noise from the read signal before normal data recovery processing is applied. However, reduction post-processing typically requires that the interfering data be read from the storage disk and stored in memory, which can increase HDD cost and complexity while also adversely impacting other performance measures such as access time.
It is also possible to reduce ITI by compensatory pre-processing of a write signal in a manner that takes adjacent bit polarities into account. Write compensation techniques of this type are disclosed in U.S. Pat. application Ser. No. 13/250,419, filed Sep. 30, 2011 and entitled “Disk-Based Storage Device having Write Signal Compensation for Magnetization Polarity of Adjacent Bits,” which is commonly assigned herewith and incorporated by reference herein.